Thursday, September 15, 2011

The Bird Feeder Antenna

Shortwave antennas are large and trying to hide one can be like trying to hide an elephant. Many listeners try different configurations, snap together antennas, wires, and flag poles to conceal their antennas. Some work out, but most get caught in the end.
First, let's look at what you can and can't have. Most restrictive housing areas have a long list of items you can't have. Clothlines, TV antennas, sheds, flags, BBQ grills, etc. And most of all, the all important shortwave antenna. It has something to do with nice surroundings.
So, what to do?
Make a list of all forbidden items and place them in a column on the left side of the page.
Next, list what you can have and list them in the center of the page.
Finally, list the items that are readly available in the area of your home. Such items are trees, gutters, vents, fences, etc. Look closely and don't leave anything out.

A hidden shortwave antenna must be just that: Hidden.
I should not be visible to the naked eye, even at a point blank range.
So, list the areas that you have a ready access to.
If you can get to the roof, write it down.
How about the attic, basement, trees, etc, especially without much notice by your neighbours.
Most shortwave antennas are discovered not because of their design or placement, but rather a neighbour spies the listener installing the antenna - or at least doing something out of the ordinary.
For example creeping around on the roof, on a Saturday afternoon, is going to draw attention. Flinging wires over trees is sure to draw some attention.

The big point here is not to install the antenna by looking like your installing an antenna or doing something out of the ordinary.
There are some new designs out on the market such as PVC vent pipe antennas for 2m. But for now we'll stick to shortwave. Most contracts for condo/PUD dwellers will allow bird feeders. And even apartment dwellers on the bottom floor can benefit from this design.

The Bird Feeder

The Bird Feeder shortwave antenna is a vertical cage wire antenna.
A What? you might ask.
Imagine if you will eight flexible wires, evenly spaced in a circular pattern, much like a ground plane. Draw these wires vertically to form a wire 'tube' and connect them together with a ring at the top. Now imagine these wires inside a telescoping PVC mast. Top that off with a bird feeder. Now, instead of a ground plane system with radials you have to put in (the neighbours are watching), you have this 'wire tube' constructed as a vertical dipole. At 10m, the Bird Feeder antenna is a mere 17' tall when raised.
What would the neighbours say?
Or for that matter the manager?
"I raised it up for the birds to get into and it keeps the squirels out too. And look, I can take it out when I'm not using it or the weather is bad. Boy, I really like watching the birds from my window. Did you know that ..."
And then go on about some rare, but not unbelievable birds that frequent the feeder. Buy a book on the subject with some pages tagged to show them what you've seen so far. At worst, they'll think you're a flake and leave you alone. Next, go in and tune up 10m and catch some rare DX instead.

What do you have to actually put into the ground? Your coax and a PVC pipe to hold the mast up. Make it low in the ground and cap it so the mower doesn't take it off. For the most part, you can leave it up.
But do remember to put some seed in the feeder once in a while...

Bird Feeder Antenna - the details

This design sounds a bit far fetched, but it works. The vertical dipole, inside the PVC push-up, is invisable. It moves up and down and can be removed without drawing attention to the fact that it contains an antenna. The flagpole design has some problems. Not all PUDs, condos, or apartments allow flagpoles. They represent a permanent fixture. Not all associations or managers are that patriotic.
Although you can pack a good vertical in a flagpole, there is the problem of radials. You can run the vertical without radials, but that's another compromise.

The design for the Bird Feeder Antenna is very simple:
You can build the vertical dipole without much trouble. The dipole consists of up to 16 wire 1/4 wave elements. There are eight on each leg of the dipole. You may use less, it's up to you. You know the bandwidth of a wire dipole and you know the band spread on 10m. If you are a general or above, you will want the extra wire elements to give you the bandwidth you need.

First, let's look at the PVC tubular mast / bird feeder support. Most hardware and home improvement center have PVC tubing. Like the steel counterparts, the mast will be graduated (large at the bottom and smaller at the top. I'm not going to recommend any sizes here as availability at your store is going to dictate what sizes you will need. I will state the the top tube should be 2" in diameter or better. You will need that size to support the bird feeder and give the mast some strength.

| | <------- Attach upper ring
| | inside
| |
|_| <------- Attach ring inside
| |

| |
| |
|___| <------- Attach ring inside
and continue two more times

The wire elements are attached in a ring format, evenly spaced, in a 360 degree pattern. Locations are noted in figure one. This allows the dipole to be folded up when the mast is lowered. More than eight wires on each leg of the dipole tends toward snags when raising and lowering.
Dipole wire element sizes are calculated by 246/f Mhz. But I recommend shorter lengths if eight elements are used. A balun can be inserted, but is not necessary. I feeder coax is needed from the dipole feed to the base of the mast. A UHF bullet (female to female) to attach your coax to.

Installing the Bird Feeder

The base piece for the bird feeder mast should be one size larger than the bottom section of the mast. The mast should be able to side in a snug fashion, but not too tight. Since you are installing a 'bird feeder', you shouldn't have any problem explaining what you are doing. Simply lay the bird feeder and mast right out in the open. Your nosey neighbors will know exactly what you're doing (almost). The coax is the tough part.
Trenches are out of the question. Use a sidewalk edger (manual) and make a thin cut in the grass. If you have other obsticles, you will have to deal with that when you come to it. The object is to do it when the neighbors are not going to notice. The thin cut in the lawn will not be seen and will 'heal' quickly. A tricky technique is to use a wheelblade on a handle. It will look like you are using a measuring device. The cable can be laid into the cut at dawn, when you are filling the bird feeder. Stepping on the cut lawn on the way back seals the cut. Now you're in business.

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The A index [ LOW is GOOD ]

  • 1 to 6 is BEST
  • 7 to 9 is OK
  • 11 or more is BAD

Represents the overall geomagnetic condition of the ionosphere ("Ap" if averaged from the Kp-Index) (an average of the eight 3-hour K-Indices) ('A' referring to amplitude) over a given 24 hour period, ranging (linearly) typically from 1-100 but theoretically up to 400.

A lower A-Index generally suggests better propagation on the 10, 12, 15, 17, & 20 Meter Bands; a low & steady Ap-Index generally suggest good propagation on the 30, 40, 60, 80, & 160 Meter Bands.

SFI index [ HIGH is GOOD ]

  • 70 NOT GOOD
  • 80 GOOD
  • 90 BETTER
  • 100+ BEST

The measure of total radio emissions from the sun at 10.7cm (2800 MHz), on a scale of 60 (no sunspots) to 300, generally corresponding to the sunspot level, but being too low in energy to cause ionization, not related to the ionization level of the Ionosphere.

Higher Solar Flux generally suggests better propagation on the 10, 12, 15, 17, & 20 Meter Bands; Solar Flux rarely affects the 30, 40, 60, 80, & 160 Meter Bands.

K index [ LOW is GOOD ]

  • 0 or 1 is BEST
  • 2 is OK
  • 3 or more is BAD
  • 5 is VERY VERY BAD

The overall geomagnetic condition of the ionosphere ("Kp" if averaged over the planet) over the past 3 hours, measured by 13 magnetometers between 46 & 63 degrees of latitude, and ranging quasi-logarithmically from 0-9. Designed to detect solar particle radiation by its magnetic effect. A higher K-index generally means worse HF conditions.

A lower K-Index generally suggests better propagation on the 10, 12, 15, 17, & 20 Meter Bands; a low & steady Kp-Index generally suggest good propagation on the 30, 40, 60, 80, & 160 Meter Bands.

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